In the past 25 years, a new type of material has entered the plastics products market. Commonly referred to as wood-plastic composites (WPCs), fiber-plastic composites, or plastic composites (PCs), the new materials have been accepted into the building products markets in applications such as outdoor decking and railing, siding, roofing and a variety of other products. The market for the wood-plastic composite has grown and WPCs now are used in automotive applications, as well as in the building products sector of the economy.
A wood-plastic composite is a blended product of wood, or other natural fibers, and a thermoplastic material. The products can be produced with traditional plastics processes such as extrusion or injection molding. For example, many building products are produced using extrusion processing similar to conventional plastics processing. The wood and plastics materials are blended before or during the extrusion process. The wood-plastic composites often compete with wood in the building products market. The current WPC materials are most often compounds of wood, or natural fibers, and polyethylene, polypropylene, or polyvinyl chloride (PVC). Presently available WPCs, however, suffer from certain drawbacks. For example, if the composite contains too high or too low of a ratio of plastic to wood, the finished product may not have the desired visual appearance or structural performance characteristics. Such products are less desirable in the marketplace. Additionally, WPCs may be expensive to produce, due to the high cost of the thermoplastic materials and other additives used in manufacture.
Ironically, many consumers expect WPCs to appear similar to wood, but also expect WPCs to perform as a robust plastic compound. To increase performance, manufacturers often incorporate UV stabilizers, antioxidants, biocides, color, fire retardants, or other additives into the WPC formulation. These additives, however, can increase manufacturing costs of the product, even though certain additives provide noticeable benefit only on a limited location on the product (e.g., in the case of UV stabilizers, the benefit only effects the exterior of the product that is exposed to sunlight). To reduce the amount of additives that are incorporated into the product, capstocking is often used. In general, capstocks are coextruded with the core material to form a thin layer of polymer over the core extruded material. Various additives may be incorporated into the capstock, thus reducing the total amount of additives per linear foot of product. These capstocks, however, may suffer from delamination from the underlying WPC and may crack or otherwise fail, causing an unsightly appearance, impaired performance, and consumer dissatisfaction.
With certain capstocks, to improve adhesion, a discrete tie layer is typically placed between the core material and capstock, but this tie layer can present a number of problems. For example, the bond formed by the tie layer may separate from one or both of the capstock and core material over time, leading to product failure. This may occur because the capstock and core material may expand and contract at different rates, due to differences in material properties, which may cause failure of the bond. Also, water, ice, or other hazards related to installed environmental conditions can still penetrate the capstock layer, for example, via gaps at the edges of discrete capstock sections. Additionally, manufacturing costs of capstocked products utilizing a discrete tie layer tend to be high, since the tie layer must be applied to finished capstock and core materials.